1. Field of the Invention
The present invention relates to an element and device for detecting internal faults in an electrical apparatus charged with an insulating gas. More particularly, the present invention relates to a device for detecting internal faults in an electrical apparatus charged with an insulating gas such as SF.sub.6 gas, by using the fact that the presence of internal faults in the insulating gas charged electrical apparatus can be detected by the generation of decomposed SF.sub.6 gas.
2. Statement of Related Art
FIG. 1 is a partial cross-sectional view illustrating a conventional internal fault detection device for an insulating gas charged electrical apparatus incorporating an internal fault element, as disclosed in, for instance, Japanese Patent Publication No. 57-38091. In FIG. 1, a hermetic grounded metal vessel 1 is provided in a part of an electrical apparatus (not shown) charged with an insulating gas such as SF.sub.6, the hermetic grounded metal vessel 1 also being filled with SF.sub.6 gas 2. A detection element supposing container 3 is provided substantially in the center of the hermetic grounded metal vessel 1. An internal fault detection element 4 constituted by a thin aluminum (Al) metal plate with a thickness of, for instance, about 5.mu.m is supported inside the element supporting container 3 in an upper portion thereof. In addition, a substance 6 containing crystalized water such as CuSO.sub.4 .cndot.5H.sub.4 O, which releases water upon coming into contact with a decomposed SF.sub.6 gas, is accommodated in a bottom portion of the element supporting container 3. A plurality of gas permeable holes 7 are provided on side surfaces of the element supporting container 3. In addition, external conductors 8 connected to an external circuit (not shown) are provided on the outside of the hermetic grounded metal vessel 1. Both ends of these external conductors 8 are electrically connected to the internal fault detection element 4 via insulated terminals 9 that pass air-tightly through a side wall portion of the hermetic grounded metal vessel 1 as well as via internal conductors 5 disposed inside the hermetic grounded metal vessel 1.
The conventional internal fault detection device is constructed as described above, and when a fault, such as a discharge, occurs in the hermetic grounded metal vessel 1, the SF.sub.6 2 is decomposed to produce an active fluorinated sulfur compound gas of SF.sub.4, SOF.sub.2, HF, etc., i.e., a decomposed SF.sub.6 gas. This decomposed SF.sub.6 gas enters the inside of the element supporting container 3 through the through holes 7 provided in the side surfaces thereof and is converted into HF upon reacting with a trace amount of water which is present in the SF.sub.6 gas 2 in the container 3, and with water and the like which is released from the substance 6 containing crystallized water. The internal fault detection element 4 produces AlF.sub.3 in a chemical reaction that takes place inside the container 3 which contains both the decomposed SF.sub.6 gas and water. That reaction EQU Al+3HF.fwdarw.AlF.sub.3 +3/2H.sub.2
Thus, a part of the internal fault detection element 4 constituted by the conductive Al metal plate is converted into nonconductive AlF.sub.3 so that the value of its resistance changes. Thus, by measuring the resistance change by an external circuit via the external conductors 8, the occurrence of any fault in the electrical apparatus can be detected.
In the internal fault detection device described above, since the internal fault detection element 4 is formed of an Al metal plate, the reaction will not proceed adequately unless a high-concentration of SF.sub.6 gas is used. Therefore, fault detection is only possible in cases where a high-concentration of decomposed SF.sub.6 gas is present, as in the case of a ground fault, but the detection speed is low. However, fault detection cannot be effected sufficiently in cases where the concentration of the decomposed SF.sub.6 gas is low as in the case of a partial discharge or local heating. Further, there arises a problem in that a material which releases water upon coming into contact with the decomposed SF.sub.6 gas, such as the crystallized water-containing substance 6, is required.
FIG. 2 is a cross-sectional view schemetically illustrating another device for detecting internal faults in an insulating gas charged electrical apparatus which is disclosed in Japanese Utility Model Laid-Open 61-40657. In FIG. 2, a hermetic vessel 11 for an electrical apparatus charged with an insulating gas, such as SF.sub.6 is provided with a movable contactor 12, a nozzle 13 provided such as to surround this movable contactor 12, a fixed contactor 14 into which the movable contactor 12 comes into contact and from which it moves away, a high-voltage conductor 16 connected to the fixed contactor 14, and a spacer 15 supporting the high-voltage conductor 16. The hermetic vessel 11 is filled with SF.sub.6 gas 2. An opening 17a of the hermetic vessel 11 is provided with a flange cover 17, on which a decomposed SF.sub.6 gas detection portion 19 having an internal fault detection element is provided. A signal from the decomposed gas detection portion 19 is supplied to an external circuit (not shown) by means of conductors 20. A gas pipe 21a for introducing the SF.sub.6 gas 2 into the hermetic vessel 11 is connected to a side pipe 21b of the hermetic vessel 11 at a gas pipe jointing portion 21.
The decomposed SF.sub.6 gas resulting from the generation of a discharge or local heating in a conducting contact portion inside the SF.sub.6 gas charged electrical apparatus moves to the decomposed SF.sub.6 gas detection portion 19 by means of convection and diffusion, and reacts with a sensitive material disposed on the decomposed SF.sub.6 detection portion 19, thereby causing a change in the electrical properties thereof. A signal representing this change in electrical properties is transmitted via the conductor 20 to a signal processing section (not shown), which in turn issues an alarm indicating that a fault is present in the electrical apparatus.
When this decomposed SF.sub.6 gas detection portion 19 is installed in an existing insulating gas charged electrical apparatus which is not provided with a decomposed SF.sub.6 gas detection portion, additional machining must be carried out so that the decomposed SF.sub.6 gas detection portion 19 can be installed on the flange cover 17.
That is, the decomposed SF.sub.6 gas detection portion 19 is conventionally fixed to the inner side of the flange cover 17, as shown in FIG. 2. Accordingly, there have been problems in that the flange of an existing insulating gas charged electrical apparatus must be machined or modified during checkups or the like so as to allow the installation of the decomposed SF.sub.6 gas detection portion 19.
In addition, since the entire processing of signals from the internal fault detection element of the conventional apparatus as shown in FIG. 1 or 2 is effected electrically, there has been other problems in that detection accuracy is low in environments where a high electromagnetic field exists.